Patients in the study all received endocrine therapy — such as tamoxifen which is the standard of care to cut the risk of cancer recurrence — for five years and were free of cancer when they stopped therapy.

But researchers found a “steady” risk of tumors recurring over the next 15 years, up to 20 years after the initial diagnosis.

“Even though these women remained free of recurrence in the first five years, the risk of having their cancer recur elsewhere — for example in the bone, liver or lung — from years five to 20 remained constant,” said senior author Daniel Hayes, professor of breast cancer research at the University of Michigan Comprehensive Cancer Center.

Women whose original tumors were large enough to have spread to four or more lymph nodes had a 40 percent risk of a cancer recurrence over the next 15 years.

For women with small, low-grade cancers and no spread to the lymph nodes, the risk of cancer recurring was 10 percent in 15 years.

“It is remarkable that breast cancer can remain dormant for so long and then spread many years later with this risk remaining the same year after year and still strongly related to the size of the original cancer and whether it had spread to the nodes,” said lead author Hongchao Pan , from the University of Oxford.

The findings raise questions about the current practice of treating women with tamoxifen or aromatase inhibitors for five years after the tumor is removed, in order to reduce the risk of a recurrence.

Some experts believe the treatment should be extended to 10 years.

Side effects can include hot flashes, vaginal dryness, osteoporosis and joint pain.

Ultimately, the decision is up to a woman and her doctor, said Hayes.

“These data can be used by patients and their health care providers as they consider whether to continue taking anti-estrogen therapy beyond five years, weighed against side effects and toxicity of the therapies,” he said.

Treatments for breast cancer have improved in recent years, so the estimated risks of recurrence may be on the high side, said co-lead author Richard Gray, of the University of Oxford.

“To assess 20-year risks, we had to study women who received their breast cancer diagnosis many years ago,” said Gray. “We know that treatments have improved since then, so recurrence rates will be somewhat lower for women who were diagnosed more recently.”